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Design and Analysis of Slotted Waveguide Antenna Radiating in a “Plasma-Shaped” Cavity of an ECR Ion Source

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Design and Analysis of Slotted Waveguide Antenna Radiating in a “Plasma-Shaped” Cavity of an ECR Ion Source Article Design and Analysis of Slotted Waveguide Antenna Radiating in a “Plasma-Shaped” Cavity of an ECR Ion Source Giorgio Sebastiano Mauro 1,* , Giuseppe Torrisi 1 , Ornella Leonardi 1 , Angelo Pidatella 1 and Gino Sorbello 1,2 and David Mascali 1 1 Istituto Nazionale di Fisica Nucleare—Laboratori Nazionali del Sud (INFN-LNS), Via S. Sofia 62, 95123 Catania, Italy; [email protected] (G.T.); [email protected] (O.L.); [email protected] (A.P.); [email protected] (G.S.); [email protected] (D.M.) 2 Dipartimento di Ingegneria Elettrica, Elettronica e Informatica, Università degli Studi di Catania, Viale Andrea Doria 6, 95125 Catania, Italy * Correspondence: [email protected] Abstract: The design of a microwave antenna sustaining a high-energy-content plasma in Electron Cy- clotron Resonance Ion Sources (ECRISs) is, under many aspects, similar to the design of a conventional antenna but presenting also peculiarities because of the antenna lying in a cavity filled by an anisotropic plasma. The plasma chamber and microwave injection system design plays a critical role in the develop- ment of future ECRISs. In this paper, we present the numerical study of an unconventionally shaped plasma cavity, in which its geometry is inspired by the typical star-shaped ECR plasma, determined by the electrons trajectories as they move under the influence of the plasma-confining magnetic field. The cavity has been designed by using CST Studio Suite with the aim to maximize the on-axis electric field, thus increasing the wave-to-plasma absorption. As a second step, an innovative microwave injection system based on side-coupled slotted waveguides is presented. This new launching scheme allows an uniform power distribution inside the plasma cavity which could lead to an increase of ion source performances in terms of charge states and extracted currents when compared to the conventional axial Citation: Mauro, G.S.; Torrisi, G.; microwave launch scheme. Finally, the use of both the “plasma-shaped” cavity and the microwave side Leonardi, O.; Pidatella, A.; Sorbello, G.; coupled scheme could make the overall setup more compact. Mascali, D. Design and Analysis of Slotted Waveguide Antenna Keywords: slotted waveguide antenna; waveguide coupler; wave-plasma coupling; resonant cavity; Radiating in a “Plasma-Shaped” Electron Cyclotron Resonance Ion Sources (ECRIS) Cavity of an ECR Ion Source. Telecom 2021, 2, 42–51. https://doi.org/ 10.3390/telecom2010004 1. Introduction and Motivation Received: 30 December 2020 Accepted: 2 February 2021 The coupling of microwaves into a plasma-filled cavity through one or more injection Published: 8 February 2021 waveguides is a fundamental aspect in Electron Cyclotron Resonance Ion Sources (ECRISs). The source performances in terms of charge states and extracted currents has been explained Publisher’s Note: MDPI stays neu- by taking into account the different patterns of the electromagnetic field that is excited tral with regard to jurisdictional clai- into the cavity, for a specific frequency (or set of frequencies) [1,2]. In order to enhance ms in published maps and institutio- the source performances, several approaches have been used in the past years, such as nal affiliations. the modification of the plasma cavity trying to better match the confining magnetic field profile [3], and the improvement of the microwave injection system by using modified or better cavity-matched waveguide configurations [4–7]. In this work, we compare the performances of different antenna systems radiating Copyright: c 2021 by the authors. Licensee MDPI, Basel, Switzerland. into cavities, possessing conventional and unconventional shapes, that can be modeled as a This article is an open access article reverberation chamber. We discuss the possible antenna configurations and we present the distributed under the terms and con- numerical design of a novel plasma cavity in which its shape has been determined by the ditions of the Creative Commons At- electron trajectories as they move under the influence of a minimum-B magnetic field profile tribution (CC BY) license (https:// (i.e., a typical configuration used in ECRISs). The modal configuration introduced into creativecommons.org/licenses/by/ the presented cavity is compared with that of a standard cylindrical cavity, for the same 4.0/). frequency interval. It will be shown that the new cavity’s shape promotes the excitation Telecom 2021, 2, 42–51. https://doi.org/10.3390/telecom2010004 https://www.mdpi.com/journal/telecom Telecom 2021, 2 43 of modes with an electric field maximum along its axis, where the plasma is typically generated. This could be advantageous in terms of plasma absorbed power, preventing the formation of the typical hollow plasma experimentally observed when the excited mode has an electric field minimum along the cavity’s axis [8]. In fact, for a circular cylinder shaped cavity, TE and TM modes are expressed in terms of Bessel functions (and first 0 derivative of Bessel functions), all equal to zero at the origin except the J0 (J1)[9]. As a second step, aiming at improving the ion source performances, we present a radically innovative microwave launching scheme that employs slotted waveguides which are side-coupled to the plasma cavity. It will be shown that this technique greatly improves the number of modes coupled inside the cavity with respect to the standard axially-coupled rectangular waveguide and at the same time makes possible a more homogeneous power transfer to the plasma thanks to the slots placed along the cavity outer wall. In addition, more space for other ancillaries is made available on the cavity’s injection flange. Due to the non-conventional shape of the novel “plasma-shaped” cavity and relative twisted slotted waveguide injection system, standard manufacturing techniques based on subtractive processes could be difficult to employ. To overcome these difficulties, for the future realization we plan to use additive manufacturing techniques, and in particular 3D metal printing through high quality metal powder compound. 2. Plasma Shaped Cavity and Axial Microwave Injection In Figure1, the “plasma-shaped” cavity is shown with its main dimensions. The cavity has diameter d = 48.5 mm and length L = 150 mm, which are typical dimensions for a second generation ECRIS plasma chamber [10]. As said, the cavity’s shape has been obtained by taking into account the electron trajectories as they move under the influence of a minimum-B magnetic field profile which is used for the plasma confinement in ECRISs [11]. Figure 1. “Plasma-shaped” cavity with its main dimensions: diameter d and length L. As first step of this numerical study, we compare the “plasma-shaped” S-parameters (i. e., the number of modes excited inside the cavity) with those of a standard cylindrical cavity usually employed in ECRISs setups. The adopted cylindrical cavity has a diameter d = 63.5 mm and a length L = 150 mm: these dimensions, comparable with those adopted for the “plasma-shaped” cavity, are the dimensions of the CAESAR Ion Source plasma cavity currently working at the INFN-LNS. Both cavities, initially fed through a standard WR62 axial waveguide, have been simulated with comparable settings: both consist of a vacuum solid with lossy metal boundary conditions ad with the same mesh quality (e.g., curvature tolerance, number of mesh refinement steps). For both cavities, we consider finite conductivity walls with s = 5.8 × 107 S/m. The two geometries with the applied mesh are visible in Figure2. Telecom 2021, 2 44 (a) (b) Figure 2. (a) Standard cylindrical cavity and (b) “plasma-shaped” cavity models. Applied mesh and axial feeding waveguides are visible. The two structures have been analyzed in terms of S-parameters. In particular, the jS11j has been evaluated in the frequency range of 14–14.5 GHz. The frequency interval has been selected in order to employ the future “plasma-shaped” cavity with the CAESAR ion source setup. The jS11j curve for the two geometries is shown in Figure3. In the case of cylindrical plasma cavity, very few and bad adapted modes are found inside the chosen frequency range (i.e., not optimal power transfer from the feeding waveguide to the cavity), while, in the case of the “plasma-shaped” cavity, more modes can be found in the same frequency range. In particular, the mode found at the frequency of 14.304 GHz presents a good waveguide-to-cavity coupling (jS11j '−15 dB) and the related electric field profile has a maximum around the cavity center, where the plasma usually forms, as seen in Figure4. 0 −5 | (dB) 11 |S −10 Cylindrical cavity Plasma-shaped cavity −15 14 14.05 14.1 14.15 14.2 14.25 14.3 14.35 14.4 14.45 14.5 Frequency (GHz) Figure 3. jS11j curve, inside the operational bandwidth of 14–14.5 GHz, for the standard cylindrical cavity and the “plasma-shaped” one. The above configurations, with the same mesh quality, can be simulated in around 20 and 30 min, respectively, with CST Studio Suite on a workstation i7 with 28 cores and 192 GB of ram. Telecom 2021, 2 45 (a) (b) Figure 4. Electric field module plot on side and front slices for: (a) cylindrical cavity ( f0 = 14.457 GHz) and (b) “plasma- shaped” cavity ( f0 = 14.304 GHz). Electric field has been normalized to the maximum value of 10 kV/m. 3. Slotted Waveguide Antenna Design With the aim to improve the microwave-to-cavity coupling and at the same time to have a more uniform and symmetric axial electric field distribution, a radically new microwave injection system has been employed. A slotted waveguide antenna operating in free space has firstly been studied, to explore its behavior as a function of the fundamental project parameters. A first preliminary antenna design is described in Reference [12].
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